At a glance, marlin and swordfish look very similar. Both species have long, sword-like bills that extend from their upper jaws. As far as the system of scientific classification goes, however, that’s where the similarities end.
Marlin belong to a group of predatory fish known as Billfish. Sailfish and spearfish also belong to this group, whereas swordfish don’t. The swordfish, or Xiphias gladius, belongs to an altogether separate group of fish, despite sharing some anatomical similarities with the Billfish.
Swordfish and marlin appear to be closely related. Not only do both species have long, vicious-looking protrusions, but they’re also large and predatory. Some even say they taste similar. However, for each similarity between the two species, there are a handful of differences.
10 Big Differences Between a Swordfish vs. Marlin
Several physical features enable us to tell the difference between a marlin and a swordfish.
In terms of size, there’s very little in it. The average blue marlin measures around 11 feet in length and weighs 200 to 400 lb, while the standard swordfish is approximately 10 feet long and 150 to 200 lb.
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In a swordfish vs. marlin comparison, the swordfish is distinguishable by its shark-like dorsal fin. Unlike the marlin, whose first dorsal fin extends along its back and past the anal fin, the swordfish’s first dorsal fin is tall and sickle-shaped, much like a shark’s.
While both species have a second dorsal fin, it’s less noticeable in the marlin due to its proximity to the tail end of the first dorsal fin.
The marlin has cleverly adapted pectoral, anal, and caudal fins that can be folded into grooves along the fish’s body to reduce drag and increase speed. The swordfish has no such adaptations, and its pectoral fins extend below its body and “increase the drag by about 21.5%” and reduce its speed accordingly.
#2 Body Shape
Although elongated, the swordfish has a round body compared to the marlin, which is more cylindrical.
Another difference between swordfish and marlin is the sword or bill itself. The swordfish’s bill is longer than swords of the billfishes and has a flattened, oval shape when viewed from the side. On the other hand, the marlin’s bill is a little shorter and, as Ernest Hemingway described it, “tapered like a rapier.”
All species of marlin are classified according to color and are generally very easy to distinguish from each other and the swordfish.
The bright blue of the blue marlin’s body is quite distinct from the more subdued colors of the swordfish, which vary from dark brown to black on the back to almost white on the belly.
A black marlin vs. swordfish comparison is a little trickier. The black marlin is also dark brown to black and paler underneath but is distinguishable by the vertical stripes that run up and down its sides.
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#4 Skin and Scales
The marlin also has a distinct hexagonal or “chickenwire-shaped” patterning on its side. This pattering is the lateral line system that enables numerous fish species to detect changes in water pressure. Only juveniles have a lateral line, making this another anatomical difference between marlin and swordfish.
Perhaps an easier way to distinguish the two is by their scales. Again, the swordfish only has scales until it reaches around 3-foot long, at the age of approximately two and a half. In contrast, marlin are “densely covered with elongate, thick, bony scales” throughout their lives.
When adult swordfish lose their scales and lateral lines, their teeth also drop out. Although juvenile swordfish have teeth, adult swordfish rely entirely on their scythe-like bills to kill their prey, which they swallow whole.
On the other hand, marlin have “small file-like teeth” on both of their jaws and the palatine, or roof of the mouth.
If you hosted a swordfish vs. marlin race, you’d soon find out which was which. When we discussed how fast a great white can swim, we discovered that the black marlin is arguably the fastest fish in the sea, with a top speed of around 130kph.
The swordfish is fast, but it doesn’t come close to the marlin with its maximum speed of 97kph.
This result is somewhat surprising given that the swordfish has a secret weapon that the marlin lacks.
A study performed in 2016 revealed that the swordfish has a large, oily gland situated at the base of its bill. This gland connects to a network of capillaries and oil-excreting pores. When combined with the denticles surrounding those pores, the oil from the gland “creates a super-hydrophobic layer that reduces streamwise friction drag and increases swimming efficiency.”
It’s still not enough to compete with the lightning-fast speeds of the marlin, though.
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#7 Diet and Feeding Techniques
Blue marlin primarily feed close to the surface, hunting pelagic fish like tuna and mackerel.
Swordfish alternate more frequently between near-surface feeding and deep-water predation.
Analyses of their stomach contents revealed a mixture of pelagic and demersal fishes, as well as squid. To have such variety in its stomach means swordfish must be utilizing its full depth range when feeding.
Based on the contents of the swordfish’s stomach, researchers were also able to establish that it uses its bill, or sword, to slash larger fish species into pieces before swallowing them. The smaller prey items were seemingly consumed whole.
This hunting technique sets the swordfish apart from the marlin, which uses its bill “to stun prey with a swift lateral strike or strikes” rather than slash it into pieces. As a marlin swims through a pod of fish, it uses its bill as a club, incapacitating its prey which it then circles back to collect and consume.
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There is a worldwide distribution of both marlin and swordfish. Both species occur in the Atlantic, Indian, and Pacific Oceans, although the swordfish appears to be more comfortable in colder water than the marlin.
Scientists believe the swordfish’s so-called “brain heater” is the reason it can cope with colder temperatures and the rapid cooling that occurs as the fish dives from the surface to depths of 2,100 feet or more.
The swordfish has a heating mechanism associated with one of its eye muscles that “insulates and warms the brain.” The marlin also has this intriguing adaptation but doesn’t appear to utilize it to the same extent as swordfish.
The swordfish and the marlin are both highly migratory. Both species typically move towards the warmer tropical waters around the equator during the winter and then back into coolers oceans in the summer.
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Similarly, they tend to utilize deeper water during the warmer daylight hours and hunt in the shallower waters at night, just as many shark species do.
A comparison of marlin vs. swordfish habitats reveals more similarities than differences. Only the swordfish’s greater tolerance of cold water distinguishes it from the marlin. There is little difference between their habitats or migratory patterns.
#9 Age and Lifespan
Swordfish reach sexual maturity when they reach five to six years of age, whereas the blue marlin matures much earlier, at just two to fours years old.
The swordfish also has a much short lifespan, living only for around 9 years. A female marlin, on the other hand, can survive for up to 20 years and a male around 10.
There are some controversies about eating both marlin and swordfish, but while we’re on the subject of feeding, let’s take a moment to compare their flavors.
Both marlin and swordfish are relatively dense fish, a little like tuna, and have a similar flavor. The flesh of the swordfish is a little lighter than that of the marlin, which has a higher fat content and a somewhat stronger flavor. The swordfish is sweeter and has a lower fat content.
Swordfish is best eaten filleted and grilled, as this helps to soften the firm flesh. It can also be pan-fried or enjoyed raw as sashimi. Marlin also lends itself to sashimi-style dishes and is also enjoyed as carpaccio and poke. Alternatively, it can be seared and served rare like a tuna steak.
Why can’t you eat a marlin fish?
While you can eat all species of marlin and swordfish, you need to take some precautions due to the high levels of toxins present in the flesh. As with all high-level predatory fish and sharks, the marlin and swordfish often contain “unhealthy levels of mercury and other toxins that may be harmful to humans.”
Marlin vs. Swordfish Comparison Table
|First dorsal fin||Shark-like dorsal fin||The dorsal fin extends along the back of the fish to the second dorsal fin|
|Pectoral Fins||Pectoral fins extend below the body||Small pectoral fins can be folded into ventral grooves|
|Pelvic Fins||No pelvic fins||Short pelvic fins that can be folded into ventral grooves|
|Shape of Snout||Long and oval-shaped||Long and round|
|Body Structure||Round and elongated||Long and cylindrical|
|Average Weight||150 to 200 lb||200 to 400 lb|
|Average length||+/- 10 feet||+/- 11 feet|
|Speed||Swordfish have a top speed of 97 kph||Black marlin can reach speeds of 130 kph|
|Taste||Light and mild||Strong flavor reminiscent of tuna|
Many people confuse the marlin with the swordfish simply because both have a long sword-like bill.
Although they share some similarities in terms of size, appearance, habitat, and diet, the marlin and swordfish are distinct species.
They don’t even belong to the same family, with the marlin being part of the Istiophoridae family and the swordfish being the only member of the unusual Xiphiidae family.
Having read this article about how to compare a marlin vs. swordfish, if you’re ever fortunate enough to catch either one, you should be able to identify the species pretty quickly based on its dorsal fin configuration, body shape, and size. If not, at least you won’t make the rookie error of mistaking it for a sailfish!
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